1. The Glutamatergic System
The amino acids glutamate and aspartate are known to function as excitatory neurotransmitters in the mammalian central nervous system. These amino acids activate a large series of excitatory receptors known as "glutamate receptors". These receptors, along with the various neurotransmitters which activate them, in turn make up the glutamatergic system, the dominant excitatory nerve impulse transmission system of the mammalian central nervous system.
Over the last few years, it has come to be recognized that the overstimulation of excitatory neurotransmitter receptors can have serious pathological consequences. The overstimulation of cultured neurons invitro by glutamate, for example, can lead to neuronal cell death. Excitotoxicity is now thought to be important in the pathogenesis of several neurodegenerative disorders, including stroke and ischemic injury.
2. The Glutamate Receptors
Both pharmacological and molecular studies have revealed that glutamate receptors comprise a large family of proteins with extensive heterogeneity at the molecular level. The different classes of glutamate receptors differ in both their preferred pharmacological ligands and in their functions.
At least three classes of glutamate receptors have been identified. One of these, the NMDA class of receptors, is specifically activated by the glutamate analog N-methyl-D-aspartate. When stimulated, NMDA receptors open ion channels and allow an influx of cations into a neuron. The non-NMDA receptors comprise another group of ionotropic receptors which mediate cation flow into neurons.
A third class of glutamate receptors which has been identified is known as the metabotropic class of receptors. These receptors are referred to as "metabotropic" because their stimulation does not result directly in the opening of an ion channel, as is the case with the NMDA and non-NMDA ionotropic glutamate receptors. Instead, the cellular effects caused by the stimulation of a metabotropic receptor are mediated by G-proteins. The metabotropic receptors can be pharmacologically distinguished from other types of glutamate receptors in that they are specifically stimulated by trans-ACPD (aminocyclopentane 1,3-dicarboxylic acid). No other known glutamate receptors are stimulated by trans-ACPD.
At least four types of metabotropic receptors have now been described (see Henley, J. M. and Johnston, G. A. R., Trends in Pharmacological Sci, 12:357-360 (1991)). Metabotropic receptors are found in nerve cells of the central nervous system both presynaptically and postsynaptically, and are also found in glial cells. In vitro, metabotropic receptors have been shown to mediate slow depolarization of hippocampal neurons. It has also been found that metabotropic receptors block spike accommodation, a slow hyper-polarization following the excitation of a neuron which limits its ability to fire action potentials. Thus, the activation of the metabotropic receptors can render neurons more excitable (see Miller, Richard J., Metabotropic excitatory amino acid receptors reveal their true colors, TIPS, 12:365-367 (1991)).
3. .beta.-Amyloid Protein and Alzheimer's Disease
Alzheimer's Disease (AD) is a progressive neurodegenerative disease which is histologically characterized by an accumulation of neuritic plaques and neurofibrillary tangles and by neuron cell death. A major component of these neuritic plaques is the .beta.-amyloid protein, which is derived from a precursor protein called the .beta.-amyloid precursor protein (APP). The .beta.-amyloid protein itself has been shown to be toxic to neurons in vitro.
The .beta.-amyloid protein has, moreover, also been shown to potentiate the cytotoxicity caused by the overstimulation of glutamate receptors. When mature neuron cultures are exposed to doses of .beta.-amyloid that are too low to cause neuronal degeneration by themselves and are then subsequently exposed to sublethal doses of glutamate, massive cell death occurs. In addition to potentiating the toxicity of glutamate, .beta.-amyloid has been shown to increase the toxicity to neurons of exposure to NMDA and kainate, which are agonists of non-metabotropic glutamate receptors (Koh, J., et al., .beta.-Amyloid protein increases the vulnerability of cultured cortical neurons to excitetoxic damage, Brain Research, 553:315-320 (1990)).
There is also evidence that .beta.-amyloid exposure specifically makes neurons more susceptible to injury or death due to the stimulation of non-NMDA glutamate receptors. The addition of 100 .mu.M of a synthetic .beta.-amyloid peptide to a neuron cell culture which had previously been exposed to quisqualate (an agonist of metabotropic receptors and also other glutamate receptors) caused a threefold increase in the release of LDH (lactate dehydrogenase, an indicator of cell damage) by the neurons in the culture. Even at higher concentrations of quisqualate, we have observed greater cell death when neurons are exposed to the .beta.-amyloid peptide compared to when they are exposed to quisqualate alone.